For Microsoft ASP.NET, I have to have Windows on my Mac for installing visual studio, then I need to use Windows for a while. But there is no such “Print Screen” key on Mac keyboard, so if I want to print a screen or a window, need to use combined key:

First, Windows needs Bootcamp installed

If want to print whole screen, use SHIFT + FN + F11

If just want to print current window, use ALT(option key) + SHIFT + FN + F11

Create a new folder and copy the files you want in the DMG image into this new folder.

Choose this new folder, and on the top menu bar click: File -> Get Info, find the size of this folder (e.g. 49.2 MB)

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Open “Disk Utility”, go to Applications -> Utilities -> Disk Utility

Click the “New Image” icon to create a new disk image, enter a name for the image, and select a size adequate for the size of your folder you created in step 2 (usually should be little bigger, e.g. 50 MB here). Set the encryption to “none” (if you don’t want set password for it) and format to “read/write disk image”

(click pics for bigger view)

Place the contents of the folder created from step 2 into the newly mounted disk image

(click pics for bigger view)

Unmount the disk image after copy done, the DMG file is what to you want

Option: Go to “Disk Utility” again, choose the image file and click “Convert”, then set “Image Format” to “read-only”.
(This step just make your disk image can’t be modified in the future)